1. Field of the Invention
The present invention relates generally to decoding Pulse Code Modulation (PCM) signals used in air traffic control radar beacon systems/selective identification feature (ATCRBS/SIF) and in airborne traffic alert and collision avoidance systems (TCAS). More specifically, this invention relates to a digital video quantizer which is used to process a PCM signal before it is decoded.
2. Description of the Related Art
The ATCRBS/SIF system comprises a ground based interrogator which transmits a query to an airborne transponder. In a TCAS system, both the interrogator and transponder are airborne and provide a means for communication between aircraft. The transponder responds to the query by transmitting a reply which is received and processed by the interrogator. The interrogator includes a receiver, an analog to digital converter (A/D), a video quantizer, a leading edge detector, and a decoder.
The reply received by the interrogator consists of a series of information pulses which may identify the aircraft, or contain altitude or other information. The reply is a PCM signal which is transmitted in either a ATCRBS format or in a Mode S format.
The ATCRBS format comprises a first and a second framing pulse with 13 information pulses positioned between them. Information is conveyed by either the presence or absence of an information pulse. The pulses have a nominal width of 0.45 microseconds, and have a spacing of 1.45 microseconds between leading edges. The leading edges of the first and second framing pulses are 20.3 microseconds apart. In some cases the second framing pulse is followed by a special position identification (SPI) pulse beginning 4.35 microseconds after the leading edge of the second framing pulse.
The Mode S format comprises a 6 microsecond preamble and a data block which is either 50 or 112 microseconds long. The preamble consists of four 0.5 microsecond pulses. The leading edges of the first and second pulses are 1 microsecond apart. The third pulse begins 3.5 microseconds after the leading edge of the first pulse, and the fourth pulse begins 4.5 microseconds after the leading edge of the first pulse. The data block begins 6 microseconds after the leading edge of the first pulse of the preamble. The data block is divided into 1 microsecond wide bit positions where each bit position contains 2 consecutive 0.5 microsecond pulse positions or chips. If a pulse is located in the first position it is considered to be set, and if it is located in the second position it is considered to be cleared.
Video quantizers used in the past were analog units which used operational amplifiers, and sample and hold devices. The operational amplifiers were used to add, subtract and compare signals, and the sample and holds were used to generate dynamic thresholds.
Analog video quantizers have several drawbacks. Using operational amplifiers to compare signals against a threshold requires the use of analog voltages to represent a threshold. Analog reference voltages require manual adjustment, and their values drift with time and temperature. In addition, thresholds based on the output of a sample and hold device have variations resulting from the decay of the sample and hold's output.